5.6 Modifying Existing List Structure

You can modify the CAR and CDR contents of a cons cell with the
primitives setcar and setcdr. We call these “destructive”
operations because they change existing list structure.

Common Lisp note: Common Lisp uses functions rplaca and
rplacd to alter list structure; they change structure the same
way as setcar and setcdr, but the Common Lisp functions
return the cons cell while setcar and setcdr return the
new CAR or CDR.